Do you really want to risk your mare's welfare over an "OOPS" inbred foal? Especially since you want to compete with your mare without having to worry about foals and foaling and torn vulvas and the age your mare is to be bred? It doesn't seem logical to me. Yes, it might be a good foal with good bloodlines and wantability, but you would most likely end up selling the foal anyway to be trained properly and to spend less money on feed. Vet bills alone from her injury could quite possible top out the money you'd get in return from a buyer. Foals are going for cheap nowadays because everyone just wants a trained horse.
I know it's hard to think about killing a foal, but think about it this way: If you could, would you reverse everything that's happened and go back to before your mare's colt bred and injured her? There would be no foal then either.
Considering the circumstances, I say abort.